Of drainage water such as rainwater flowing into a sewerage system laid in an urban area, part is stored in or discharged into the earth by facilities for storing or infiltrating rainwater, while the rest is discharged to rivers. Solids such as sediment, a variety of garbage, papers, and fallen leaves are mixed in the rainwater drainage flowing in the sewerage system after dropping onto the ground surface and flowing on the ground surface, and if those solids are discharged to the rivers, they will cause water contamination; if they flow into the rainwater storing-infiltrating facilities, it will become necessary to frequently perform maintenance of the facilities; therefore, they will cause disadvantage in terms of cost. Furthermore, rainwater is temporarily stored underground without removal of solids in a rainwater storage tank or the like for flood countermeasures against torrential rain, and the stored rainwater needs to be pumped up during fine weather to be discharged to a river; on that occasion, however, the solids are also simultaneously discharged to the river, so as to cause the river water contamination and environment pollution problem. As means to avoid the water contamination and environment pollution, a separator of drainage water to separate the solids is provided in part of the sewerage system.
It is common practice to use a screen or a filter as a device for separating and removing the solids in drainage water, but they are likely to be clogged by solids, to cause a maintenance problem thereof. A solution to this problem is a separator configured to generate a horizontal swirling flow (swirl) by energy of the drainage water influent into a separation tank, without use of power such as electricity, and to separate the solids by the swirling flow, and the known separator of this type is, for example, trade name FliudSep available from UFT Inc., Germany. However, while the separator permits easy maintenance because of no use of the filter or screen, it is difficult for the separator to perform secure separation and capture of floating and fine solids of size to be removed.
On the other hand, Patent Literature 1 describes a separator as a combination of the swirling flow generating method and the screen separation method. The separation tank in Patent Literature 1 is configured as follows: a cylindrical screen is disposed in the lower part of the separation tank with a circular plane cross section, drainage water is supplied in a tangent direction from the upper part of the separation tank to generate a swirling flow in the tank, solids are separated by the screen disposed in the lower part, and only the drainage water is made to pass to the outside of the screen.
Patent Literature 1: Japanese Patent Application Laid-open No. 8-141326